! Proposal 6572, submission 1 ! PI: Francesco Paresce ! Received Mon Mar 11 11:05:59 EST 1996 ! From: gdmarchi@eso.org ! +====================+ ! | PED OUTPUT PRODUCT | ! +====================+ ! ! /home/mc9a/science/gdmarchi/proposals/6572.prop ! Generated by PREPROCESSOR, version 6.0e ! Date: Fri Feb 23 15:30:14 MET 1996 Proposal_Information Title: Searching for Low Mass Stars: the Mass Function at the H Burning Limit Proposal_Category: GO Scientific_Category: STELLAR POPULATIONS Cycle: 6 Investigators PI_Name: Francesco Paresce PI_Institution: European Southern Observatory CoI_Name: Guido De Marchi CoI_Institution: Space Telescope Science Institute Contact: CoI_Name: Georges Meylan CoI_Institution: European Southern Observatory Contact: CoI_Name: Martino Romaniello CoI_Institution: Scuola Normale Superiore Contact: CoI_Name: Ralph Bohlin CoI_Institution: Space Telescope Science Institute Contact: CoI_Name: Roberto Buonanno CoI_Institution: Osservatorio di Roma Contact: CoI_Name: Francesca D'Antona CoI_Institution: Osservatorio di Roma Contact: CoI_Name: Vittorio Castellani CoI_Institution: Universita di Pisa Contact: CoI_Name: Enzo Brocato CoI_Institution: Osservatorio di Teramo Contact: Abstract: This is the natural continuation of our Cycle-5 program aimed at deriving the luminosity and mass function of very low mass stars (VLM) in globular clusters (GCs). Cycle-5 observations, scheduled for 1995 November, will study three clusters all with the same metallicity, yet with completely different dynamical histories (both internal and due to the interaction with the Galaxy) to investigate the effects of dynamics on the present day mass function at the VLM end. We propose now to use the same technique (deep WFPC2 observations in the F606W and F814W bands) to analyze four clusters with very similar dynamical evolution, but spanning a large range of metallicity. Recent studies show that metallicity may affect the star formation process in GCs, resulting in a steeper IMF at low masses for metal-poor objects, although a flattening below ~ 0.3 Msun is expected in all cases. A thorough intercomparison of the luminosity and mass functions of several different clusters will provide useful tests on VLM stars evolution, will deliver long awaited hints on the origin of the dark matter, and will improve the understanding of the dynamical processes at work in globular clusters. As a byproduct, several cluster white dwarfs will be detected and their cooling sequences will be investigated. Questions Observing_Description: We propose the use of the WFPC2 to obtain V and I band images of four globular clusters to study the low mass end of the luminosity and mass functions. We have selected our sample of clusters (listed in Table 1) in such a way that the effects of metallicity on the IMF of the stellar population can be investigated for VLM stars through the analysis of their present-day mass function. Therefore, we have selected four clusters with similar dynamical state (all of them are relaxed) and dynamical histories (destruction times, column 3 in units of T_ Hubb.), but with different values of the metallicity (column 4). Since we already have observed the VLM luminosity and mass function of two low-metallicity GCs (Fe/H~eq -1.9), and since our Cycle 5 program (to be executed in 1995 October) will provide observations of three more such clusters, we have selected here objects with higher metal content, by choosing two GCs with Fe/H~eq -0.5 and two with Fe/H~eq -1.0. Although, in principle, one object per metallicity class would suffice, the second pair is added to allow for a consistency check and to improve the statistics. Nonetheless, to minimize the overall exposure time clusters' distances have been kept as close as possible (distance moduli (m-M)_V in column 2), compatibly with the selection criteria. begintablet begintabularlcccccccc multicolumn9cTable1: Physical parameters of the selected clusters and required exposure times, multicolumn1cCluster &(*) &(^+) &(*) &(*) &(*) & t_ f606w & t_ f814w & Total, multicolumn1lNGC & multicolumn1c(m--M)_V & multicolumn1cT_d & multicolumn1cFe/H & multicolumn1cr_hl & multicolumn1cr_c& multicolumn1c(min)& multicolumn1c(min)& multicolumn1cOrb., 6838 & 13.8 & 2.9 & -0.58 & 127" & 38" & 90 & 125 & 5 , 6723 & 14.8 & 2.6 & -1.09 & 86" & 57" & 300 & 340 & 15 , 6352 & 14.5 & 3.0 & -0.51 & 117" & 50" & 170 & 210 & 9 , 6171 & 15.0 & 3.7 & -0.99 & 104" & 32" & 460 & 500 & 23 , endtabular, (*) Djorgovski (1993), (^+) Aguilar, Hut & Ostriker (1989) endtable Our strategy is to observe near the half-mass (~ half-light) radius, where the present day MF is expected be as close as possible to the IMF (Richer et al. 1991). In order to derive the MF we need to convert the LF (the observed data) by the use of the M-L relation. This is a very delicate issue, which has plagued previous determinations of the MF in GCs, because of the uncertainties associated with the M-L itself at these low masses. Models developed by one of us (FD), however, appear today to proceede secure and find confirmation in the observational data (D'Antona 1995; D'Antona & Mazzitelli 1995). We should, nevertheless, notice that the cross-comparison of the LFs themselves is already very significant, as we have accurately selected objects for which the dynamical history (both internal and resulting from the Galactic tidal field) is expected to be the same. As dynamics is the only process that can modify the IMF in time, having selected clusters with a similar dynamical evolution would ensure that any differences found in the LFs originate in the IMFs, in that the latter were different at birth due to metallicity. Columns 7 and 8 show the exposure times needed to reach, for each region in each cluster, the low end of the MS with the F606W and F814W broad band filters, while column 9 gives the total required number of orbits for each cluster. These exposure times have been estimated on the basis of our past experience with the same WFPC2 filters: we have used our previous observations of NGC 6397, M 15, and 47 Tuc taken with the same instrumental set-up and same scientific purpose (Paresce et al. 1995; De Marchi & Paresce 1995a, 1995b) and have scaled the corresponding exposure times proportionally to the distance of each cluster, in order to reach the low end of the MS at M_I ~eq 12 still with SNR ~eq 5 (see Figure 1). Our experience shows, in fact, that a SNR of ~ 5 in the PSF peak is mandatory to efficiently identify faint stars and to guarantee accurate photometric measurements. Below this threshold, dark current fluctuations, as well as hot pixels and cosmic rays not properly removed make detection very problematic and photometry increasingly less reliable (i.e. incomplete). The exposure times calculated in this way are in fair agreement with those obtained by running the ``on-line'' WFPC2 simulator available on the STScI WWW server. Isolated pixels with a peak SNR of 3 or more surrounded by pixels at (or near) the local background level are often encountered (especially when the detector is operated above -80 ^oC). The severe PSF undersampling of the WF2--4 detectors makes it difficult to decide unambiguously whether these structures are real stars or dark current fluctuations or even cosmic ray residuals. To partly overcome this inconvenience, we propose to apply a few (5--10) pixel offset (dithering) between consecutive images taken of each region in each filter. Based on our own experience and on that of other colleagues, this is the most efficient way of recovering faint stars with reliability and eliminate high frequency noise, thus providing complete and accurate photometry of low mass MS stars. Improving the sampling will also make it easier to discriminate, through profile fitting, between stars and faint galaxies, which are expected to be a statistically significant source of contamination at V> 26 (Bahcall, J.N., Guhathakurta, P., and Schneider, D.P. 1990, Science, 248, 178; Paresce et al. 1995a). We do not require any special calibration, since standard PIPELINE procedures for dark current, bias, and flat field calibration have proven most adequate to our purposes. The standard IRAF digiphot.apphot aperture photometry package, as well as the digiphot.daophot and ROMAFOT PSF-fitting photometry packages for more crowded fields, will be used for data reduction. Real_Time_Justification: As our HST WFPC2 observations of NGC 6397 (Paresce et al. 1995a), M 15 (De Marchi & Paresce 1995a), and 47 Tuc (De Marchi & Paresce 1995b) have shown, the LFs of VLM MS stars as derived from ground-based data are plagued by a number of observational uncertainties (Hesser 1993). The observational difficulties inherent in the project that we propose here make the HST the only instrument, at present, capable of reaching the scientific objectives of our investigation. Calibration_Justification: Additional_Comments: Fixed_Targets Target_Number: 1 Target_Name: NGC6352 Alternate_Names: Description: STELLAR CLUSTER, GLOBULAR CLUSTER Position: RA=17H 25M 29.2S +/- 0.1S,DEC=-48D 25' 22" +/- 1",PLATE-ID=0689 Equinox: 2000 RV_or_Z: RA_PM: Dec_PM: Epoch: Annual_Parallax: Flux: V = 28 B-V = 1.8 E(B-V) = 0.28 Comments: V MAGNITUDE AND B-V COLOR REFER TO THE FAINTEST STARS WE EXPECT TO DETECT Target_Number: 2 Target_Name: NGC6496 Alternate_Names: Description: STELLAR CLUSTER, GLOBULAR CLUSTER Position: RA=17H 59M 2.0S +/- 0.1S,DEC=-44D 15' 54" +/- 1",PLATE-ID=061A Equinox: 2000 RV_or_Z: RA_PM: Dec_PM: Epoch: Annual_Parallax: Flux: V = 28 B-V = 1.8 E(B-V) = 0.28 Comments: V MAGNITUDE AND B-V COLOR REFER TO THE FAINTEST STARS WE EXPECT TO DETECT Solar_System_Targets Generic_Targets Scan_Data Visits Visit_Number: 01 Visit_Requirements: On_Hold_Comments: Visit_Comments: Exposure_Number: 11 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 40S Special_Requirements: POS TARG 83,83 Comments: Exposure_Number: 12 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2400S Special_Requirements: SAME POS AS 11 EXPAND MAX DUR 150% Comments: Exposure_Number: 13 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 40S Special_Requirements: SAME POS AS 11 Comments: Exposure_Number: 14 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2360S Special_Requirements: SAME POS AS 11 EXPAND MAX DUR 150% Comments: Exposure_Number: 15 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2760S Special_Requirements: SAME POS AS 11 EXPAND MAX DUR 150% Comments: Exposure_Number: 16 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 30S Special_Requirements: SAME POS AS 11 Comments: Exposure_Number: 17 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2760S Special_Requirements: SAME POS AS 11 EXPAND MAX DUR 150% Comments: Exposure_Number: 18 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 30S Special_Requirements: SAME POS AS 11 Comments: Exposure_Number: 19 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2760S Special_Requirements: SAME POS AS 11 EXPAND MAX DUR 150% Comments: Exposure_Number: 20 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 30S Special_Requirements: SAME POS AS 11 Comments: Exposure_Number: 21 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2760S Special_Requirements: SAME POS AS 11 EXPAND MAX DUR 150% Comments: Exposure_Number: 22 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 30S Special_Requirements: SAME POS AS 11 Comments: Exposure_Number: 23 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2760S Special_Requirements: SAME POS AS 11 EXPAND MAX DUR 150% Comments: Exposure_Number: 24 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 30S Special_Requirements: SAME POS AS 11 Comments: Exposure_Number: 25 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2760S Special_Requirements: SAME POS AS 11 EXPAND MAX DUR 150% Comments: Exposure_Number: 26 Target_Name: NGC6352 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 30S Special_Requirements: SAME POS AS 11 Comments: Visit_Number: 02 Visit_Requirements: On_Hold_Comments: Visit_Comments: Exposure_Number: 31 Target_Name: NGC6496 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 40S Special_Requirements: POS TARG 83,83 Comments: Exposure_Number: 32 Target_Name: NGC6496 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2300S Special_Requirements: SAME POS AS 31 EXPAND MAX DUR 150% Comments: Exposure_Number: 33 Target_Name: NGC6496 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 30S Special_Requirements: SAME POS AS 31 Comments: Exposure_Number: 34 Target_Name: NGC6496 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 40S Special_Requirements: SAME POS AS 31 Comments: Exposure_Number: 35 Target_Name: NGC6496 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2360S Special_Requirements: SAME POS AS 31 EXPAND MAX DUR 150% Comments: Exposure_Number: 36 Target_Name: NGC6496 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 30S Special_Requirements: SAME POS AS 31 Comments: Exposure_Number: 37 Target_Name: NGC6496 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2760S Special_Requirements: SAME POS AS 31 EXPAND MAX DUR 150% Comments: Exposure_Number: 38 Target_Name: NGC6496 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2760S Special_Requirements: SAME POS AS 31 EXPAND MAX DUR 150% Comments: Exposure_Number: 39 Target_Name: NGC6496 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2760S Special_Requirements: SAME POS AS 31 EXPAND MAX DUR 150% Comments: Exposure_Number: 40 Target_Name: NGC6496 Config: WFPC2 Opmode: IMAGE Aperture: WFALL Sp_Element: F606W Wavelength: Optional_Parameters: CR-SPLIT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 2760S Special_Requirements: SAME POS AS 31 EXPAND MAX DUR 150% Comments: Data_Distribution ! 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